There are already known various methods of and arrangements for testing optical reflecting surfaces for shape accuracy, among them such utilizing interferometric evaluation of a laser beam which has been reflected from the reflecting surface being tested. The heretofore known methods and arrangements become more and more complex with increasing complexity of the shape of the reflecting surface to be tested. So, for instance, it is extremely difficult to obtain reliable metrological measurements with respect to concave grazing incidence hyperboloidal or paraboloidal reflecting surfaces that are to be employed, for instance, in X-ray telescopes used in astronomy or the like. Because of their configuration, solid bodies provided with reflecting surfaces of this kind will be referred-to herein as barrel optics, but it is to be understood that the present invention is not limited to such optics.
It would be possible, at least theoretically, to test the reflecting surface of the barrel optics by directing a laser beam through a lens arrangement against the reflecting surface so as to reach this surface at the same incidence as that encountered during the normal use of the barrel optics, or at the opposite incidence. Then, the laser beam reflected from the reflecting surface could be interfered, for instance in a Fizeau-type interferometer, with the original laser beam and the resulting interferogram would then be evaluated in order to determine the extent and distribution of any deviations of the actual shape of the surface being tested from its ideal or desired shape.
Aside from alignment issues and system component errors, as well as difficulties in optically acting on the laser beam in such a manner that the wave front returning from the reflecting surface, after picking up any deviations of such surface, interferes with the original wave front in the desired manner, one important problem which severely limits the usefulness of this technique is that it can hardly be used, if at all, in the context of fabricating barrel optics and particularly their reflecting surfaces. This is so because great difficulties are encountered in localizing surface configuration errors when testing at angles of incidence that exhibit a high degree of obliqueness with respect to the surface being tested, because of the considerable and varying foreshortening occurring under these testing conditions. As a consequence, opticians or other personnel attempting to correct the configuration errors will be severely hampered in their efforts to bring the tested surface into its desired form, and there is a pronounced danger that such personnel will take erroneous corrective actions because of incorrectly evaluating or interpreting the interferogram due to the effects of the aforementioned foreshortening and other factors.
In view of these difficulties, the approach currently used most often for testing such concave barrel optics is purely mechanical in nature, that is, it employs a mechanical profilometer. This profilometer performs two kinds of measurements: one in the circumferential direction and the other in the axial direction. However, since it would be too time-consuming and cumbersome to obtain a fine grid of data sets, only a very limited number of each such measurements is taken, and interpolation is performed to obtain surface deviation values intermediate the actually obtained data in the circumferential direction and in the axial direction, respectively. This technique leaves much to be desired in terms of accuracy and reliability not only because of the disregarding of those actual deviations that occur within the grid between the measured and interpolated locations, but also because inherent properties of or environmental influences on the mechanical testing structure, be it wear, temperature changes, vibrations or other phenomena, influence the profilometer stylus point position and thus the measurement accuracy, and because the physical contact of the stylus point with the surface being tested may even damage such surface.
Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art.
More particularly, it is an object of the present invention to provide a method of testing concave reflecting surfaces, especially relatively complex ones such as those of barrel optics, for surface accuracy, which method does not possess the disadvantages of the known methods of this type.
It is yet another object of the present invention to devise a method of the above kind which would make it possible to improve the accuracy and determinativeness of the indication of any imperfections of the surface being tested.
A concomitant object of the present invention is to develop an arrangement which is particularly suited for the performance of the above method.
Still another object of the present invention is to design the arrangement of the type here under consideration in such a manner as to be able to eliminate the influence on the final testing results of any aberrations that are not those of the surface being tested.
An additional object of the present invention is so to construct the arrangement of the above type as to be relatively simple in construction, inexpensive to manufacture, easy to use, and yet reliable in operation.